1. Field of the Invention
The present invention relates to a combustor of a gas turbine and especially relates to a combustor of a gas turbine which is characterized by a staging method of fuel.
2. Description of the Prior Art
The outline of a conventional combustor of a gas turbine will be described hereinafter. FIG. 18A and FIG. 18B are schematic block diagrams showing the construction of a conventional combustor of a gas turbine; and FIG. 18A is a longitudinal cross-sectional view thereof and FIG. 18B is a figure viewed from the downstream side. As shown in FIG. 18A and FIG. 18B, a combustor of a gas turbine comprises a transition piece 10 being provided with an inner space as a combustion chamber and a combustor basket 2 being provided with a mechanism for producing air-fuel pre-mixture, wherein a pilot nozzle 3 being connected to a pilot cone 5 is installed in the center of axis of the combustor basket 2. Main nozzles 4 being connected to main burners 6 serving as premixing tools are installed in the circumferential portion of the pilot nozzle 3, and in an embodiment of the present invention, eight main nozzles are installed equiangularly.
In addition, a pilot swirl 7 is installed between the pilot cone 5 and the outer circumference in the vicinity of the tip of the pilot nozzle 3; and main swirls 8 are installed between the main burners 6 and on the outer circumference of the vicinity of the tips of the main nozzles 4. Moreover, by installing a flat plate 4a to the side surface of the main nozzle 4 on the upstream side of the main swirl 8, a flat plate type of nozzle is employed, having fuel injection holes provided on the surface thereof. A combustor 1 is constructed as described above.
Main fuel being supplied to the main nozzles 4 produces air-fuel pre-mixture in the main burners 6. On the other hand, pilot fuel being provided to the pilot nozzle 3 generates pilot flame (diffusion flame) by the pilot nozzle 3. Then, the air-fuel pre-mixture is injected to the transition piece 10 and ignited by the pilot flame in the transition piece 10, generating a premixed flame inside the transition piece 10. In addition, a bypass elbow 9 is installed so as to protrude from the outer circumference surface of the transition piece 10 to the casing side, and a bypass valve “BV” is installed to the tip thereof.
For the rest, a combustor of a gas turbine which uniforms the mixture of the air and the fuel gas in the radial direction in the main nozzles and reduces the amount of diffusion combustion in the pilot combustion chamber so as to advance reduction of NOx is disclosed in the Patent Application Laid Open No. H6-137559. Additionally, a combustion equipment of a gas turbine which has high combustion efficiency although combustion is partial so as to increase the ratio of premixed combustion generating a small amount of NOx as well as which can achieve stable combustion when the density of fuel of the air-fuel pre-mixture is low and achieve combustion with NOx reduced in a wide load zone is disclosed in the Patent Application Laid Open No. H8-14565.
Conventionally, for a combustor of a gas turbine, stable combustion and combustion in a low environmental load have been searched for in a wide range of load condition from a partial load to a full load. However, because the conventional combustor of a gas turbine as described hereinabove applies lean pre-mixed combustion due to reduction of NOx, the fuel is relatively diluted in order to achieve low combustion temperature at the time of partial load, resulting in generation of a large amount of unburned portion of the fuel. Reduction of the unburned portion of the fuel at the time of partial load is an important issue for the market needs.
Therefore, in order to reduce such unburned portion of the fuel as described hereinabove, the operational parameters are set in a manner that the pilot fuel ratio is set high and the bypass valve is opened. However, the upper limit of the pilot fuel ratio is limited by the fuel pressure, and also the upper limit of the ratio of fuel versus air is limited in the combustion area due to the size of the bypass valve. Moreover, because in the existing operational mode, fuel is supplied to all the main nozzles (eight nozzles in the above-mentioned example of a conventional combustor) and the pilot nozzle (one nozzle) since start-up, naturally, reduction of the unburned portions comes to be limited if nothing is done.
Additionally, the conventional control method of combustion has a tendency to deteriorate the property of exhaust gas and generate combustion vibration and further, an increase in metal temperature of the combustor when the load is low, which needs to be improved.